Electronic computing devices have become increasingly important to data computation, analysis and storage in our modern society. Modern direct access storage devices (DASDs), such as hard disk drives (HDDs), are heavily relied on to store mass quantities of data for purposes of future retrieval. As such long term data storage has become increasingly popular, and as the speed of microprocessors has steadily increased over time, the need for HDDs with greater storage capacity to store the increased amount of data has also steadily increased.
Consequently, there are seemingly constant development efforts to improve the areal density of the media implemented in hard disk drives, where the areal density is measured as the product of bits per inch (“BPI) and tracks per inch (“TPI”). BPI refers to the number of bits that can be written and later reread per linear inch along a track, whereas TPI refers to the number of individual tracks per radial inch. Advancements in areal density, however, are not limitless in conventional magnetic recording. Consequently, thermally assisted recording techniques are developing.
Thermally Assisted Recording (TAR)
Thermally assisted data recording is motivated by limitations in the areal density possible in conventional magnetic recording, known as the superparamagnetic limit. That is, traditional scaling of magnetic grain size will not be possible in the very near future due to random thermal switching of the grains. For written data to be thermally stable for a period of several years (at about 330° K), the minimum size of a magnetic grain is limited to approximately 8 nm. Although materials exist with a minimum stable size of approximately 2 nm, the coercivity of these materials is higher than the maximum attainable field that can be produced by the write head. In order to use high coercivity materials it will be necessary to temporarily heat the media while it is being written. The heating temporarily lowers the coercivity so that magnetic data bits may be orientated by the write head. Heat must be confined to a single data track in order to prevent accidental erasure of adjacent tracks.
With thermally assisted recording, read back of the data is accomplished in the conventional manner. Because the ultimate areal density limit in magnetic recording is determined by the grain size at a given number of grains per bit, thermally assisted recording will permit areal data densities about 10 times higher than are possible in conventional magnetic recording.
With thermally assisted recording techniques, heating from a near-field optical source temporarily lowers the coercivity of the media so that magnetic data bits may be orientated by the write head. Bits are “set” when the coercivity of the media is less than the applied field. Because the dynamic coercivity of the media drops with temperature, the sharpness of the magnetic transition and, therefore, the in-track bit density, is determined by a combination of the media temperature gradient at the trailing edge of the heated region and the magnetic field gradient. The temperature gradient is likened to a field gradient according to dH0/dx=(dT/dx)*(dH0/dT), where H refers to coercivity at very short time-scales, x to distance, and T to temperature. Ideally, to achieve the highest total effective field gradient and the sharpest magnetic transitions in the media, the trailing edge thermal gradient from the near-field optical source and the gradient from the magnetic write head should overlap. In general, this is difficult to achieve because the magnetic pole pieces must allow for an optical path to the near-field source without large optical losses.
For TAR to be effectively realized, it will be necessary to confine heat to a single data track approximately 50 nm wide or smaller, with high efficiency. Candidate near-field optical sources typically use a low-loss metal (Au, Ag, Al, Cu) shaped in such a way to concentrate surface charge motion at a tip apex located at the slider air bearing surface (ABS) when light is incident. Oscillating tip charge creates an intense near-field pattern, heating the disk. Sometimes, the metal structure can create resonant charge motion (surface plasmons) to further increase intensity and disk heating. For example, when polarized light is aligned with the corner of a triangular-shaped gold plate, an intense near field pattern is created at that corner. Resonant charge motion can occur by adjusting the triangle size to match a surface plasmon frequency to the incident light frequency.
Another near-field transducer is the ridge slot waveguide from microwave circuits applied to optical frequencies (also known as the “c-aperture”). This shape is characterized by five parameters, including the metal thickness. Light polarization is aligned with the ridge and incident light concentrates surface charge at the tip of the ridge. Previously, a ridge waveguide in silver has been optimized at a wavelength of 516 nm and a metal-to-metal fly-height of 8 nm. Furthermore, far field measurements obtained for various c-aperture sizes indicate a spectral shift, while narrow resonant behavior has been observed when a pattern of ridges is used to excite surface plasmons around a long slot waveguide and enhance far field transmission.